Diffusion bonding is one of methods for bonding together stainless steel materials, and stainless steel diffusion-bonded products assembled by diffusion bonding are applied to various uses, such as a heat exchanger, mechanical parts, parts for a fuel cell, parts for an electrical appliance, parts for a plant, members constituting a decorative article, and building materials. As methods for diffusion bonding, there are an “insert material insertion method” in which an insert material is inserted into the interface between materials to be bonded to cause solid phase diffusion or liquid phase diffusion so as to bond together the materials, and a “direct method” in which the surfaces of stainless steel materials are directly contacted with each other and diffusion-bonded together.
As an insert material insertion method, there have conventionally been known many techniques, for example, a method using dual-phase stainless steel as an insert material (Patent Literature 1), a liquid-phase diffusion bonding method using an insert material in a foil form being plated with Ni and Au several μm thick and having the same composition as that of the material to be bonded (Patent Literature 2), and a method using austenitic stainless steel containing Si in a large amount in the range of 11.5% or less as an insert material (Patent Literature 3). Further, “brazing” using a nickel based brazing material (for example, JIS: BNi-1 to 7) or a copper based brazing material as an insert material can be regarded as a kind of liquid-phase diffusion bonding. These insert material insertion methods have an advantage in that diffusion bonding can be surely and relatively easily achieved. However, as compared to the direct method, the insert material insertion methods are disadvantageous in that the use of an insert material increases the cost, and in that the bonding portion made of a metal different from the materials bonded may cause the corrosion resistance to become poor.
On the other hand, generally, the direct method is unlikely to achieve a satisfactory bonding strength, as compared to the insert material insertion method. However, the direct method has the capability to advantageously reduce the production cost, and hence various studies have also been made on the direct method. For example, Patent Literature 4 discloses a technique in which stainless steel materials having a S content of 0.01% or less in the steel are subjected to diffusion bonding in a non-oxidizing atmosphere at a predetermined temperature so as to avoid deformation of the materials, thus improving the diffusion bonding properties of the stainless steel materials. Patent Literature 5 discloses a method in which a stainless steel foil material having a surface roughened by an acid pickling treatment is used. Patent Literature 6 discloses a method in which stainless steel having an Al content suppressed so that an alumina film which is a factor in inhibiting diffusion bonding is unlikely to be formed during the diffusion bonding is used as a material to be bonded. Patent Literature 7 discloses that diffusion is promoted by using a stainless steel foil having deformation imparted thereto by cold working. Patent Literature 8 has a description of ferritic stainless steel having an appropriate composition for direct diffusion bonding.